Elucidating the Mechanism of Uranium Mediated Diazene NN Bond Cleavage

Investigation into the reactivity of reduced uranium species toward diazenes has revealed key intermediates in the four-electron cleavage of azobenzene. Trivalent Tp*2U­(CH2Ph) (1a) (Tp* = hydrotris­(3,5-dimethylpyrazolyl)­borate) and Tp*2U­(2,2′-bpy) (1b) both perform the two-electron reduction of diazenes affording η2-hydrazido complexes Tp*2U­(AzBz) (2-AzBz) (AzBz = azobenzene) and Tp*2U­(BCC) (2-BCC) (BCC = benzo­[c]­cinnoline) in contrast to precursors of the bis­(Cp*) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide) ligand framework. The four-electron cleavage of diazenes to give trans-bis­(imido) species was possible by using Cp*U­(MesPDIMe)­(THF) (3) (MesPDIMe = 2,6-((Mes)­NCMe)2-C5H3N, Mes = 2,4,6-trimethylphenyl), which is supported by a highly reduced trianionic chelate that undergoes electron transfer. This proceeds via concerted addition at a single uranium center supported by both a crossover experiment and through addition of an asymmetrically substituted diazene, Ph-NN-Tol. Further investigation of 3 and its substituted analogue, Cp*U­(tBu-MesPDIMe)­(THF) (3-tBu) (tBu-MesPDIMe = 2,6-((Mes)­NCMe)2-p-C­(CH3)3-C5H2N), with benzo­[c]­cinnoline, revealed that the four-electron cleavage occurs first by a single electron reduction of the diazene with the redox chemistry performed solely at the redox-active pyridine­(diimine) to form dimeric [Cp*U­(BCC)­(MesHPDIMe)]2 (5) and Cp*U­(BCC)­(tBu-MesPDIMe) (6). While a transient pyridine­(diimine) triplet diradical in the formation of 5 results in H atom abstraction and p-pyridine coupling, the tert-butyl moiety in 6 allows for electronic rearrangement to occur, precluding deleterious pyridine-radical coupling. The monomeric analogue of 5, Cp*U­(BCC)­(MesPDIMe) (7), was synthesized via salt metathesis from Cp*UI­(MesPDIMe) (3-I). All complexes have been characterized by 1H NMR and electronic absorption spectroscopies, X-ray diffraction, and, where pertinent, EPR spectroscopy. Further, the electronic structures of 3-I, 5, and 7 have been investigated by SQUID magnetometry.